This invention relates to an exposure method, an exposure apparatus and/or a device manufacturing method. More particularly, the invention concerns a projection exposure method, a projection exposure apparatus and/or a device manufacturing method using the same, suitably usable in a lithographic process for manufacture of semiconductor devices, liquid crystal display devices or thin film magnetic heads, for example, for transferring a mask pattern to a substrate through a projection optical system.
For further improvements in integration and miniaturization of a semiconductor device, further increases of resolving power and transfer accuracy are required in exposure apparatuses which bear the lithography. In order to meet this requirement, exposure amount control for ensuring that a resist applied to a wafer (substrate) is exposed with a correct exposure amount should be carried out very precisely.
As regards light sources usable in photolithography, there are i-line (365 nm), KrF (248 nm), ArF (193 nm) and, in near future, F2 (157 nm). As for exposure methods, there are a step-and-repeat method and a step-and-scan method.
In an example of exposure amount control method to be used with the step-and-repeat method, an exposure amount is measured between shot exposures by, using a sensor provided inside an illumination optical system. The thus obtained measured value is multiplied by a transmission factor of an optical system (a portion of an illumination system and a projection optical system) from the sensor to the wafer, to determine a predicted exposure amount on the wafer. On the basis of this, the exposure amount for a subsequent shot is adjusted by using a light quantity adjusting means, such as a laser output or ND filter, for example.
In an example to be used with the step-and-scan method, during scan of each shot, the exposure is made by plural light pulses. During this period, the output (light quantity) of a light source is continuously measured by use of a sensor inside an illumination optical system, and each pulse output is adjusted so that the integrated exposure amount in the shot reaches a predetermined level (corresponding to an optimum exposure amount determined by a resist). Also in this case, like the step-and-repeat method, the transmission factor of an optical system (a portion of an illumination system and a projection optical system) from the sensor to the wafer is estimated as a certain value, and the exposure amount on the wafer is predicted by multiplying the output by this estimated value.
These examples however involve the following inconveniences. That is, in these methods, the transmission factor of the optical system from the sensor inside the illumination system to the wafer (including a portion of the illumination system and the projection system) is assumed as a constant value. Alternatively, variation in transmission factor with respect to the exposure amount is measured beforehand, and the results are inputted. Anyway, such procedure has to correct the exposure amount while excluding a condition which cannot be disregarded in practice.
Practically, if the exposure wavelength becomes shorter than 200 nm, the absorption of light by an optical material (quartz, fluorite, optical coating, or the like) becomes large, and it applies a substantial influence to the precision of exposure amount control. For example, it has been found on the basis or data obtained by basic researches made to quartz that the laser projection causes both short-term degradation and long-term degradation, that the absorption is once lowered (recovery of transmission factor) in response to suspension of light irradiation, while, in response to restart of light irradiation, the absorption regresses to an original absorption curve (regression of absorption). Therefore, in exposure apparatuses, the transmission factor may vary very complicatedly in accordance with the exposure condition such as laser pulse energy, pulse number, reticle transmission factor, exposure suspension, or the like.
It is accordingly an object of the present invention to provide an exposure method, an exposure apparatus and/or a device manufacturing method, by which the transmission factor of an optical system, including a projection optical system, can be measured during an exposure process such that the exposure amount can be controlled very precisely.
In accordance with an aspect of the present invention, there is provided an exposure apparatus, comprising: an illumination optical system for illuminating a pattern with light from a light source; a projection optical system for projecting light from the pattern onto a substrate to be exposed, to thereby expose the substrate with the pattern; and measuring means for measuring, when exposure light contributable to the exposure of the substrate is being projected onto the substrate, a portion of the exposure light passing through at least a portion of said projection optical system but not reaching the substrate, to thereby detect a change in transmission factor of at least one of optical elements of said illumination optical system and said projection optical system.
In one preferred form of this aspect of the present invention, the exposure apparatus may further comprise control means for controlling the exposure amount on the basis of the measurement by said measuring means.
The at least one of the optical elements of said illumination optical system and said projection optical system may refer to at least one optical element of said illumination optical system and at least one optical element of said projection optical system.
The apparatus may further comprise storing means for storing an initial value of measurement, and comparing means for comparing a result of measurement made after the initial value is measured, and the initial value, wherein a change in transmission factor may be detected on the basis of the comparison.
The apparatus may further comprise control means for controlling the exposure amount on the basis of the detection of the change in transmission factor.
The projection optical system may include a refractive optical element, wherein the portion of the exposure light to be measured by said measuring means may be light reflected by a surface of the refractive optical element of said projection optical system.
The measurement of the portion of exposure light by said measuring means may be carried out by receiving light reflected by the surface of the refractive optical element with use of a first light receiving element which may be disposed one of inside said projection optical system, just below the mask, just above the mask, and inside said illumination optical system.
The first light receiving element may be placed in an optically conjugate relation with a slit aperture plane inside said illumination optical system.
The first light receiving element may be placed in a relation of a pupil plane with respect to a slit aperture plane inside said illumination optical system.
The surface of the refractive optical element may have a transmission factor made lower as compared with another refractive optical element of said projection optical system.
The refractive optical element may have an anti-reflection film formed on its surface, wherein an anti-reflection function of the anti-reflection film may be lowered as compared with another light transmitting surface.
The transmission factor of the surface of the refractive optical element may be lowered by applying a metal coating to the surface.
The surface of the refractive optical element may be disposed close, as much as possible, to the substrate.
The surface of the refractive optical element may be disposed at such position that light reflected by the surface of the refractive optical element is not excessively diverged or converged along a reflection light path.
A portion of the light reflected by the surface of the refractive optical element may be extracted outwardly of a path of the exposure light, by a reflection mirror disposed at a light path of said projection optical system.
The surface of the refractive optical element may be disposed at such position that the light reflected thereby is collected on the reflection mirror.
A portion of the light reflected by the surface of the refractive optical element may be extracted outwardly of a path of the exposure light, by a reflection mirror disposed at a light path of said illumination optical system.
A portion of the light reflected by the surface of the refractive optical element may be extracted outwardly of a path of the exposure light, on the basis of transmission of the same through a reflection mirror disposed at a light path of said illumination optical system.
The apparatus may further comprise storing means for storing an initial value of measurement, and comparing means for comparing a result of measurement made after the initial value is measured, and the initial value, wherein a change in transmission factor may be detected on the basis of the comparison.
The apparatus may further comprise control means for controlling the exposure amount on the basis of the detection of the change in transmission factor.
The apparatus may further comprise a second light receiving element for measuring a portion of illumination light emitted from said light source and directed through said illumination optical system toward the pattern.
The portion of the illumination light may be extracted on the basis of reflection of the same by a reflection mirror disposed inside said illumination optical system.
The portion of the illumination light may be extracted on the basis of transmission of the same through a reflection mirror disposed inside said illumination optical system.
The apparatus may further comprise a first light receiving element for receiving and measuring light reflected by the refractive optical element, and a second light receiving element for receiving and measuring a portion of illumination light emitted from said light source and directed through said illumination optical system toward the pattern, wherein a change in transmission factor may be detected on the basis of the results of measurements made by said first and second light receiving elements.
The apparatus may further comprise storing means for storing an initial value of measurement, and comparing means for comparing a result of measurement made after the initial value is measured, and the initial value, wherein a change in transmission factor is detected on the basis of the comparison.
An integrated exposure amount may be calculated on the basis of the detection of the change in transmission factor.
An exposure amount may be controlled on the basis of the detection of the change in transmission factor.
An exposure amount may be controlled on the basis of the detection of the change in transmission factor.
An integrated exposure amount may be calculated on the basis of detection of the change in transmission factor, and an exposure amount may be controlled on the basis of the calculation of the integrated exposure amount.
In accordance with another aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: applying a resist to a surface of a substrate to be exposed; and exposing the substrate by use of an exposure apparatus as recited above.